This invention relates to an electrically variable transmission for a motor vehicle powertrain, and more particularly to an electrically variable transmission having variable ratio input split differential gearing and independently shifted power paths through the transmission.
An electrically variable transmission (EVT) utilizes one or more electric machines (which may be operated as motors or generators) and a differential gearing arrangement to provide a continuously variable ratio drive between input and output. An EVT is particularly useful in hybrid electric vehicle powertrains including an engine that is directly coupled to the transmission input and also including an electrical storage battery used to supply power for propulsion and to recover energy from braking. An EVT is also particularly useful with an engine that is designed for constant speed operation.
Such a powertrain is shown and described in the U.S. Pat. No. 5,931,757 to Schmidt, issued on Aug. 3, 1999, and assigned to the assignee of the present invention. In Schmidt, the electric machines are conveniently arranged on a common axis with a compound planetary differential gearset, and operation over a wide output speed range without requiring undesirably high electric machine speeds is achieved by using multiple friction clutches to reconfigure or shift the operating mode of the gearset at a mid-range output speed. Increasing the number of operating modes will improve the powertrain efficiency and minimize the capacity, cost and weight of the electric machines. However, the number of operating modes in prior art EVT configurations have been minimized due to considerations of cost, gearing complexity, size, weight and so on.
To minimize clutch energy and output torque disturbances during shifting, the clutches in the Schmitt configuration are operated synchronously xe2x80x94that is, with substantially zero relative speed between the two sides of each clutching mechanism. Another way to minimize clutch energy is to interrupt the power flow through the transmission, as has been very well known in manual automotive transmissions, and in the automated manual transmission (AMT). To avoid interrupting the power flow, an AMT can be configured with multiple power paths, where one power path remains active while the other shifts; however, such configurations typically require duplication of many parts.
Accordingly, what is desired is an efficient EVT configuration with multiple power paths and multiple operating modes, and that is smaller and lower in cost and complexity than presently known EVT configurations.
The present invention is directed to an improved EVT configuration including a mechanical power path from input to output through input split differential gearing, a first electric machine that regulates the ratio between input and output, a partly electrical power path to the output, and a second electric machine that functions as an electric power balancer. Both power paths are independently reconfigurable to provide multiple operating modes to reduce the torque and speed requirements of the second electric machine, and to prevent power interruption during shifting. For example, during reconfiguration of the mechanical power path, an external electrical source such as a storage battery can supply torque to the output through the second electric machine.
Preferably, reconfiguration of the electrical power path is performed when the torque produced by the second electric machine is substantially zero. If electrical power is neither drawn from nor supplied to the EVT, the second machine will produce zero torque when the speed of the first electric machine is substantially zero. If electrical power is drawn from or supplied by a storage battery or other external source, the second electric machine will produce zero torque when the power required by the first electric machine is equal to the power that can be supplied by the external source.
Preferably, reconfiguration of the power paths is achieved with clutches having sliding splines or other compact means typically found in the manual transmission and in the AMT. In the illustrated embodiment, a system of overlapping sliding splines is used to select all of the configurations of differential gearing available from a single planetary gearset.
In a preferred implementation, the EVT of the present invention includes first and second independently reconfigurable multiple-mode gearsets serially coupled by a connecting shaft, and features automated manual transmission gearing for reduced cost, size and complexity. The first gearset differentially couples an input and the first electric machine to the connecting shaft, and the second gearset couples the connecting shaft to an output. The second electric machine is coupled to the second gearset with multiple-mode AMT gearing to provide power balancing, and independently reconfiguring the gearsets and the coupling of the second machine multiplies the number of overall transmission operating modes to provide improved efficiency without significantly increasing the cost, size and complexity of the transmission, and without sacrificing the advantages of known EVT configurations.